A recent development in telecommunications technology has been the introduction of the asynchronous transfer mode (ATM) transmission technique. The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, e.g. voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a header indicating its destination. The service traffic is adapted typically into 53 byte cells comprising 0.5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently.
A description of a cell multiplexing apparatus handling multiple items of information is given by Takashima et al. in specification U.S. Pat. No. 5,509,007. An arrangement for providing ATM call interworking with the PSTN is described by Zsehong et al. in IEEE/ACM Transactions on Networking, vol. 2, No. 1, 1 Feb. 1964, pages 30 to 39. Voice transport on an ATM broadband network is described by Covington et al. in Communications Technology for the 1990's and Beyond, Dallas, Nov. 27-30, 1989, vol. 3 of 3, 27 Nov. 1989, IEEE, pages 1921 to 1925.
Various enhancements of the basic ATM transmission protocol have been proposed to accommodate specific user requirements. One of these enhancements has been the introduction of minicells for low bit rate users to reduce the cell assembly delays previously experienced by such users. In such a system, minicells from a number of users can be multiplexed together and packed into a standard ATM cell for transmission over a common virtual channel. A number of recommendations have been made for an adaptation layer to provide support of these services, but none of these has effectively accommodated the different requirements of the system users.
There are three major types of system user are detailed below.